Fuel cell system and process for controlling the same

ABSTRACT

A fuel cell system comprises a fuel cell, which generates power by supplying anode gas and cathode gas into the fuel cell, a compressor, which controls the amount of the gas to be supplied into the fuel cell, and a pressure control valve, which controls the gas pressure of the fuel cell. The pressure control valve is provided on the downstream of the fuel cell. The fuel cell is controlled by changing an amount of the supply gas by the compressor, and thereafter changing the opening of the pressure control valve during the transition period of the fuel cell.

RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No.2000-223194 filed on Jul. 25, 2000 in Japan. The contents of theaforementioned application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a fuel cell system which generateselectricity through the reaction between hydrogen and oxygen, andprocess for controlling the fuel cell in the fuel cell system. Moreparticularly, the invention relates to a fuel cell system havingimproved performance during the transition period, at which the powergeneration amount is changed, and a process for producing the same.

2. Description of Related Arts

In recent years, an electric powered vehicles each carrying variousdriving motors instead of the conventional engine has been developed.One example of such types of electric powered vehicles includes a fuelcell carried vehicle having a Proton Exchange Membrane Fuel Cellabbreviated as “PEM FC” (hereinafter PEM type fuel cell or simplyreferred to as fuel cell) as a power source for a driving motor, andsuch PEM type fuel cell carried vehicles have been sharply developed.

PEM FC comprises a stack structure having a lot of single cells, whichare power generation units, laminated on each other. Each cell has aconfiguration composed of an anode side separator having a hydrogenpassage, a cathode side separator having an oxygen passage, and amembrane-electrode assembly (hereinafter abbreviated as “MEA”)intervened between these separators. MEA is composed of a protonexchange membrane abbreviated as PEM, each surface of PEM with acatalyst layer and a gas diffusion layer laminated one after another(one surface having an anode side catalyst layer and a gas diffusionlayer and the other having a cathode side catalyst layer and a gasdiffusion layer).

In such PEM FC, when hydrogen gas flows through the hydrogen passagefrom the inlet side to the outlet side of the anode and when air (as anoxidant gas) flows through the oxygen passage from the inlet side to theoutlet side of the cathode, the protons permeate through PEM of MEA in awet state from the anode side of each cell, migrating to the cathodeside. This causes each cell to generate electromotive force ofapproximately 1 V. In PEM FC having such a power generation mechanism,air and hydrogen are continuously supplied to continue the powergeneration. Consequently, an air intake system, which compresses air,for example, by a compressor is provided at the inlet side of cathode,and an air exhaust system, for example, having a backpressure controlvalve, is provided on the outlet side of cathode. In addition, ahydrogen gas supply system, which supplies hydrogen by an ejector, isprovided on the inlet side of anode.

As described above, in the fuel cell system having the air supplysystem, the air exhaust system, and the hydrogen supply system providedon the fuel cell, the revolution speed of the compressor is controlledto be increased or decreased by increasing or decreasing an amount ofthe air flowing to the cathode inlet, whereby a power generation amount(output current or output power) is controlled (increased or decreased).At this time, if the pressure difference between the poles, i.e., thedifference between the hydrogen pressure and the air pressure, becomesunduly large, there is a fear of breaking PEM making up MEA.Consequently, the hydrogen gas pressure at the anode inlet side and theair pressure at the cathode inlet side are separately controlled so thatthe pressure difference between the poles falls within a tolerancerange. Specifically, in the conventional fuel cell system, therevolution speed of the compressor is controlled to be a target valuewhere the air-flow amount to the cathode inlet side is controlled to bea target air flow amount, and the opening of the backpressure controlvalve of the air exhaust system is controlled so that the air pressurebecomes a target pressure.

Meanwhile, it takes a very short period that the opening of thebackpressure control valve reaches a target value in comparison with theperiod that the air pressure reaches a target air pressure. However, inthe conventional fuel cell system, the opening of the backpressurecontrol valve is sharply controlled so as to be a target valuecorresponding to the target airflow amount. For example, as shown inFIG. 5, when the airflow amount Q is increased to a given target airflowamount QT, the opening γ of the backpressure control valve is sharplycontrolled to be a target value corresponding to the target airflowamount QT as shown in the broken line. For this reason, at thetransition period until the airflow amount Q reaches the target airflowamount QT, the backpressure control valve is excessively wide-opened tothe target opening corresponding to the target airflow amount QT inadvance and, thus, the pressure P of the air to be compressivelytransferred toward the cathode inlet by a supercharger is escaped towarddownstream of the backpressure control valve. As a result, the airpressure P at the cathode inlet side is once decreased and then reachesa target air pressure PT, conducting that the pressure increase isdelayed. The behavior at the time where the airflow amount Q isdecreased to a given target airflow amount QT is that the air pressure Pis once increased, and then reaches the target air pressure PT duringthe transition period, delaying the decreasing of the pressure.

In the conventional fuel cell system as described above, during thetransition period when the airflow amount at the cathode inlet side isincreased or decreased to a target airflow amount corresponding to thedecreasing or increasing of the power generation amount, the airpressure at the cathode inlet side is once decreased or increased.Accordingly, there poses a problem that pressure difference between thepoles in the fuel cell system (pressure difference between the anodeside and the cathode side applied to PEM of MEA) is increased. Also, theconventional fuel cell system is disadvantageous in that there is a timelag until the air pressure at the cathode inlet reaches a target airpressure, leading to poor responsibility.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention is to provide afuel cell system and a process for controlling the same, which canprevent the increasing of the pressure difference between the poles inthe fuel cell during the transition period when the airflow amounttoward the cathode inlet reaches a target airflow amount, and which cansolve a time lag until the air pressure at the cathode inlet sidereaches a target air pressure.

According to the present invention, there is provided a process forcontrolling a fuel cell comprising a fuel cell, which generates power bysupplying anode gas and cathode gas into the fuel cell, a compressorwhich controls the amount of the gas to be supplied into the fuel cell,and a pressure control valve which controls the gas pressure of the fuelcell and which is provided on the downstream of the fuel cell,

said process comprising:

changing an amount of the supply gas by said compressor during thetransition period of said fuel cell, and thereafter, changing theopening of said pressure control valve.

Also, there is provided a process for controlling a fuel cellcomprising:

a flow amount feedback control step which controls the flow amount ofthe gas supplied into a fuel cell to be a prescribed value; and

a pressure feedback control step which controls the pressure of the fuelcell to be a prescribed value,

said feedback steps being stopped during the transition period of thefuel cell.

According to the present invention a fuel cell system is also provided,which comprises a fuel cell, which generates power by supplying anodegas and cathode gas into the fuel cell,

a compressor which controls the amount of the gas to be supplied intothe fuel cell,

a pressure control valve which controls the gas pressure of the fuelcell and which is provided on the downstream of the fuel cell,

airflow control means, which controls the airflow toward the cathodeinlet side to be a target airflow amount corresponding to a target powergeneration amount of the fuel cell by controlling the revolution numberof said compressor, and

air pressure control means, which controls the air pressure at thecathode inlet to be a target air pressure corresponding to the targetairflow amount by controlling the opening of said pressure control valveat the stationary state, and which controls the pressure control valvecorresponding to the change in the airflow amount detected from saidflow sensor to thereby control the air pressure to be the target airpressure during the transition period.

In the fuel cell system according to the present invention, said airpressure control means during the transition period preferably controlsthe opening of said pressure control valve depending upon the airflowamount detected from the flow sensor and upon the target air pressure.

Also, in the fuel cell system according to the present invention, saidair pressure control means during the transition period is preferablykept operating until said airflow amount reaches the target airflowamount.

Furthermore, according to the present invention, there is also provideda process for controlling a fuel cell comprising controlling the powergeneration amount of the fuel cell by controlling the flow amount andthe pressure of the air compressively transferred into the cathode inletside of the fuel cell, and

controlling said air pressure to be a target airflow amountcorresponding the detected airflow amount, which is gradually changed,during the transition period of said fuel cell.

According to the process for controlling the fuel cell of the presentinvention and the fuel cell system of the present invention, whichcontrols the amount of the supply gas and thereafter controls theopening of the pressure control valve, the increasing of the pressuredifference between the poles within the fuel cell can be prevented,ensuring the prevention of the damage of PEM making up MEA of the fuelcell. What is more, the time delay by which the air pressure at thecathode inlet side reached the target air pressure can be solved,improving the response to the increasing or decreasing of the powergeneration amount of the fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of a vehicle drive line including afuel cell carried on a vehicle according to one embodiment of thepresent invention.

FIG. 2 is a functional block diagram of a control system of a fuel cellsystem according to one embodiment of the present invention.

FIG. 3 is a graph showing the relation between a target airflow amountand a target air pressure in the fuel cell system according to oneembodiment of the present invention.

FIG. 4 is a schematic flowchart diagram illustrating the steps performedto determine whether the target power generation amount has changed.

FIG. 5 is a graph showing control characteristics during the course ofthe transition in the fuel cell system according to one embodiment ofthe present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of a fuel cell system and a process for controlling the fuelcell system according to the present invention will now be described byreferring to the attached drawings in which FIG. 1 is a configurationdiagram of a vehicle drive line including a fuel cell carried on avehicle according to one embodiment of the present invention, FIG. 2 isa functional block diagram of a control system of a fuel cell systemaccording to one embodiment of the present invention, and FIG. 3 is agraph showing the relation between a target airflow amount and a targetair pressure in the fuel cell system according to one embodiment of thepresent invention.

First, the configuration of a vehicle drive line of a vehicle having afuel cell system according to the present invention carried thereon willnow be described by referring to FIG. 1. The vehicle is a so-called anelectric vehicle which carries a fuel cell (FC) 3 as an electric powersource for a power driving motor (traction motor) (EVM) 2, whichrotatably drives a traveling wheel 1. The fuel cell (FC) 3 has a circuitconfiguration that the fuel cell (FC) 3 loads power to a power drivingunit (PDU) 5 and a battery 6 via a high-pressure distributor (DC/DC) 4.The power driving unit 5 has a circuit configuration so that the powerdriving unit 5 at least drives the power driving motor 2 describedabove, and a driving motor 7F for a supercharger (S/C) 7B, which servesas a compressor and which will be described fully later on.

The fuel cell (FC) 3 is a PEM type fuel cell having a plurality ofcells, each of which is a power generation unit, laminated thereon. Thefuel cell (FC) 3 has an air intake system 7, which supplies air (oxygen)to a cathode inlet side, and an air exhaust system 8, which dischargesthe air from the cathode outlet side. Also provided on the fuel cell(FC) 3 is a hydrogen intake system 9, which supplies hydrogen gas to ananode inlet side.

On the air intake system 7 of the fuel cell (FC) 3, from the upstreamside toward the downstream side, an air cleaner (A/C) 7A, thesupercharger (S/C) 7B, and an intercooler (heat exchanger) (H/C) 7C areplaced. The air intake system 7 has a flow mater 7D, which detects anamount of air flowing toward the cathode inlet side, provided on theupstream of the supercharger (S/C) 7B, and a pressure sensor 7E, whichdetects an air pressure around the cathode inlet of the fuel cell (FC)3. Any air type of cleaner can be used as the air cleaner (A/C) 7, aslong as it has a function of filtering the flowing air. It is alsopossible to provide an air-intake silencer such as a resonator at theupstream of the air cleaner (A/C) 7A.

The supercharger (S/C) 7B can be driven at a rotation speed ranging from0 to 12,000 rpm, and can linearly change an airflow amount Q dependingupon the rotation speed. The supercharger (S/C) 7B is driven in arotatable manner through driving current supplied at a given duty ratiosupplied from the drive unit (PDU) 5, whose rotation number iscontrolled in a variable manner at a changing ratio of 12,000 rpm.

The intercooler (H/E) 7C serves as a heat exchanger, which allows thepower driving motor (EVM) 2, the fuel cell (FC) 3, the high-pressuredistributor (DC/DC) 4, the power driving unit (PDU) 5, and the drivingmotor 7F for cooling in a thermally exchangeable manner through acoolant.

Examples of the flow sensor 7D, which can be mentioned, include varioustypes of airflow meters (airometers), such as vane type, Karman vortextype, and hot-wire type airflow meters. As the pressure sensor 7E, anappropriate type pressure sensor, such as a semiconductor type pressuresensor, can be used.

The air exhaust system 8 of the fuel cell (FC) 8 has a backpressurecontrol valve 8A for controlling air pressure P at the cathode inletside of the fuel cell (FC) 3. The backpressure control valve 8A has a CVvalue (capacity of valve) of about 8.5 and the valve-opening speed ofapproximately 8,000 degree/sec. In the backpressure control valve 8A,the opening is controlled at a interval of 10 ms.

On the other hand, from the upstream toward the downstream, the hydrogenintake system 9 of the fuel cell (FC) 3 has a hydrogen tank 9A, apressure control valve 9B, and ejector 9C. It is noted that the hydrogenintake system 9 is configured so that the hydrogen gas remaining unusedin power generation and exhausted from the anode outlet is recycled intothe ejector 9C.

The fuel cell system according to one embodiment of the presentinvention has a control unit 11 which at least inputs detected signalsfrom the flow sensor 7D, the pressure sensor 7E, an accelerator sensor10, which will be described fully later on, and which outputs controlsignals to the power driving unit (PDU) 5, the backpressure controlvalve 8A, and the pressure control valve 9B, respectively. The controlunit 11 is composed of, as hardware, an input/output interface I/Obetween the control unit 11 and the flow sensor 7D, the pressure sensor7E, the accelerator sensor 10, the backpressure control valve 8A, thepressure control valve 9B, etc., and an analog/digital (A/D) converter,which converts analog signals input from the flow sensor 7D, thepressure sensor 7E and the accelerator sensor 10 to digital signals,Read Only Memory (ROM), which memorizes various data and programs, aswell as Random Access Memory (RAM), which temporarily memorizes variousdata, Central Processing Unit (CPU), which executes various calculation,and the like.

As the software configuration, the control unit 11 is composed ofairflow control means (program), which controls the flow amount Q of theair flowing toward the cathode inlet side of the fuel cell (FC) 3 to bea target airflow amount corresponding to a target power generationamount of the fuel cell (FC) 3 by controlling the rotation speed of thesupercharger (S/C) 7B, and air pressure control means (program), whichcontrols the air pressure P at the cathode inlet to be a target airpressure corresponding to the target airflow amount through controllingthe opening of the backpressure control valve 8A. Also, contained in thecontrol unit 11 is hydrogen gas pressure control means (program), whichcontrols the hydrogen gas pressure at the anode inlet side to be atarget hydrogen pressure corresponding to the target air pressurethrough controlling the opening of the pressure control valve 9B.Particularly, the control unit 11 according to this embodiment has airpressure control means during the transition period. Specifically,during the transition period that the airflow amount Q is graduallychanged toward the target airflow amount according to the change in atarget power generation amount, the opening of the backpressure controlvalve 8A is controlled one after another corresponding to the change inthe airflow amount Q, which is detected by the flow sensor 7D one afteranother whereby air pressure control means, which controls the airpressure P during the transition period is controlled one after anotheris configured.

In order to attain the functions of the airflow amount control means,the air pressure control means, the hydrogen gas pressure control means,and the air pressure control means during the transition period, thecontrol unit 11 has respective functional blocks as shown in FIG. 2.Specifically, as a block for attaining the function of the airflowamount control means, the control unit 11 possesses a unit 11A forsetting a target power generation amount, a unit 11B for setting atarget airflow amount, a unit 11C for feedback-controlling an airflowamount, and a unit 11D for outputting a control signal for controllingthe power driving unit. As the block for attaining the function ofcontrolling the air pressure control means, the control unit 11possesses a unit 11E for setting a target air pressure, a unit 11F forfeedback-controlling the air pressure, a unit for instructing theopening of the backpressure control valve, a unit 11J for switching theinput, a unit 11K for outputting a control signal for controlling theopening, and a unit 11H for setting the opening the backpressure controlvalve. The control unit 11 also possesses as the block for attaining thefunction of the hydrogen pressure control means, a unit 11L for settingthe opening of the pressure control valve and a unit 11M for outputtinga control signal for controlling the opening of the pressure controlvalve.

Each of the functional blocks possessed by the control unit 11 will nowbe described. To the unit 11A for setting a target power generationamount is input a signal α of an accelerator angle from an acceleratorsensor 10. The accelerator sensor 10 is composed, for example, of apotentiometer, which detects an angle α of the accelerator pedal (notshown) to be stepped in corresponding to the change in the load of thefuel cell carried vehicle. The angle α of the accelerator pedal outputfrom the accelerator sensor 10, which is an analog signal, is thenconverted into a digital signal, which is input to the unit 11A forsetting a target power generation amount. The unit 11A for setting atarget power generation amount makes a map research for seeking a targetpower generation amount corresponding to the signal of the acceleratorangle α, and a signal concerning the researched target power generationamount IT is output to the unit 11L for setting the opening of thepressure control valve.

Based upon the signal concerning the researched target power generationamount IT input from the unit 11A for setting a target power generationamount, the unit 11B for setting a target airflow amount makes a mapresearch for seeking an airflow amount required for attaining the targetpower generation amount IT, and outputs the signal concerning the targetairflow amount QT to the unit 11C for feedback-controlling an airflowamount and the unit 11E for setting a target air pressure.

To the unit 11C for feedback-controlling an airflow amount are input thesignal concerning a target airflow amount QT from the unit 11B forsetting a target airflow amount, and the airflow amount Q detected fromthe flow sensor 7D, converted from the analog data into digital data.The unit 11C for feedback-controlling an airflow amount outputs to theunit 11D for outputting a control signal for controlling the powerdriving unit, a PID actuation signal QC for rapidly converging thedeviation of the airflow amount Q to the target airflow amount QT uponzero by carrying out proportion, integration, or differentiation.

The unit 11D for outputting a control signal for controlling the powerdriving unit produces a pulse width modulation signal (PWM signal) forcontrolling the current for running through the driving motor 7F throughpulse modulation based upon the PID actuation signal QC, and output itto the power driving unit (PDU) 5.

The power driving unit (PDU) 5 is rotates the driving motor 7E atdriving current with a prescribed polarity and a prescribed duty ratioby switching operation of a bridging circuit of a power field effecttransistor (FET) (not shown) based upon the PID actuation signal QC fromthe unit 11D for outputting a control signal for controlling the powerdriving unit. Specifically, the driving motor 7E is rotated so that thesupercharger (S/C) 7B attains the target airflow amount QT. Although thedetail description is omitted, the power driving unit (PDU) 5 iscomposed so as to rotatably drive the power driving motor (tractionmotor) (EVM) 2 at current with a prescribed duty ratio based upon thesignal of the accelerator angel α of the accelerator sensor 10.

The unit 11E for setting a target air pressure makes a map research forseeking an air pressure required for attaining the target airflow amountQT within the CV value of the backpressure control valve 8A based uponthe signal concerning the target airflow amount QT, with reference tothe map having characteristics as shown in FIG. 3. The target QTsearched as described above is then output to the unit 11F forfeedback-controlling the air pressure, the unit 11H for setting theopening the backpressure control valve, and the unit 11L for setting theopening of the pressure control valve.

To the unit 11F for feedback-controlling the air pressure are input thesignal concerning a target air pressure PT from the unit 11E for settinga target air pressure, and the air pressure P output from the pressuresensor 7E, converted from the analog data into digital data. The unit11F for feedback-controlling the air pressure outputs to a unit 11G forindicating the opening of the backpressure control valve a PID actuationsignal PC for rapidly converging the deviation between the air pressureP and the target pressure PT upon zero by carrying out proportion (P),integration (I), or differentiation (D).

Based upon the PID actuation signal PC, the unit 11G for indicating theopening of the backpressure control valve makes a map research forseeking an opening of the backpressure control valve 8A required forobtaining the target air pressure PT within the CV value of thebackpressure control valve 8A, and the signal β for indicating theopening is output to the unit 11J for switching the input.

To the unit 11H for setting the opening the backpressure control valve,the signal concerning the target air pressure PT from the unit 11E forsetting a target air pressure is input, as well as the signal of theairflow amount Q from the flow sensor 7D is input at every 10 ms cycle.Based upon the signal of the target air pressure and based upon thesignal of the airflow amount Q, which is gradually changed toward thetarget airflow amount Qt, the unit 11H for setting the opening thebackpressure control valve makes a map research for seeking an openingof the backpressure control valve 8A required for attaining the targetair pressure PT one after another at an interval of 10 ms within the CVvalue of the backpressure control valve 8A, and the resulting signal yfor setting the opening of the valve is output to the unit 11J forswitching the input, while altering the signal γ for setting the openingof the valve at an interval of 10 ms.

To the unit 11J for switching the input, the signal β for indicating theopening from 11G for indicating the opening of the backpressure controlvalve and the signal γ for setting the opening of the valve from theunit 11H for setting the opening the backpressure control valve areinput, and the signal of the airflow amount from the flow sensor Q andthe signal of the target airflow amount from the unit 11B for setting atarget airflow amount are also input thereto. By comparing the airflowamount Q with the target airflow amount QT, the unit 11J for switchingthe input outputs the signal γ for setting the opening of the valve fromthe unit 11H for setting the opening the backpressure control valve tothe unit 11K for outputting a control signal for controlling the openingduring the course of the transition period until the airflow amount Treaches the target airflow amount QT, while the unit 11J for switchingthe input outputs the signal β for indicating the opening from the unit11G for indicating the opening of the backpressure control valve to theunit 11K for outputting a control signal for controlling the openingunder the stationary conditions after the airflow amount Q reaches thetarget airflow amount QT.

The unit 11K for outputting a control signal for controlling the openingoutputs a driving signal D1 having a given polarity and a given dutyratio in order to PWM control the opening of the backpressure controlvalve 8A produced based upon the signal γ for setting the opening of thevalve from the unit 11H for setting the opening the backpressure controlvalve or the signal β for indicating the opening to the unit 11G forindicating the opening of the backpressure control valve.

Based upon the target air pressure PT from the unit 11E for setting atarget air pressure, the unit 11L for setting the opening of thepressure control valve makes a map research for seeking a targethydrogen gas pressure slightly greater than the target air pressure,also makes a map research for seeking an opening of the pressure controlvalve 9B required for attaining the target hydrogen gas pressure, andoutputs the signal δ for setting the opening of the valve to the unit11M for outputting a control signal for controlling the opening of thepressure control valve.

The unit 11M for outputting a control signal for controlling the openingof the pressure control valve outputs a driving signal D2 having a givenpolarity and a given duty ratio in order to PWM control the opening ofthe pressure control valve 9B produced based upon the signal δ forsetting the opening of the valve to the pressure control valve 9B.

In the fuel cell system according to the first embodiment configured asdescribed above, when the power generation amount of the fuel cell (FC)3 is increased, for example, if the accelerator pedal (not shown) isstepped in, the accelerator sensor 10 outputs an accelerator openingsignal α corresponding to the amount of the accelerator to be stepped into the control unit 11. The control unit 11 then controls the flowamount and the pressure of the air compressively transferred to thecathode side according to the change in the target power generationamount of the fuel cell (FC) 3 to be the target airflow amount QT andthe target air pressure, respectively, as shown in the flowchart of FIG.3, whereby the power generation amount of the fuel cell (FC) 3 iscontrolled to be the target power generation amount. In this case,during transition period that the airflow amount Q is gradually changedtoward the target airflow amount QT, the air pressure P is controlled tobe a target air pressure QT one after another corresponding to thegradual change in the airflow amount Q.

In the control unit 11, the unit 11A for setting a target powergeneration amount, which inputs the signal α of the accelerator openingfrom the accelerator sensor makes a map research for seeking a targetpower generation amount IT corresponding to the signal a of theaccelerator opening (S1), and outputs the produced signal to the unit11B for setting a target airflow amount. Subsequently, the unit 11B forsetting a target airflow amount make a map search for seeking the targetairflow amount required for attaining the target power generation amountIT (S2), and outputs the produced signal concerning the target airflowamount QT to the unit 11C for feedback-controlling an airflow amount andthe unit 11E for setting a target air pressure, respectively. The flowsensor 7D detects the airflow amount Q at the cathode inlet side of thefuel cell (FC) 3 (S3), and outputs the detected signal to the unit 11Cfor feedback-controlling an airflow amount, the unit 11H for setting theopening the backpressure control valve, and the unit 11J for switchingthe input, respectively.

Subsequently, in order to converge the detected airflow amount Q to thetarget airflow amount QT, the unit 11C for feedback-controlling anairflow amount, the unit 11D for outputting a control signal forcontrolling the power driving unit, and the power driving unit (PDU) 5execute feedback control of the revolution number of the supercharger(S/C) 7B (S4). Specifically, the unit 11C for feedback-controlling anairflow amount, which has input the signal concerning the target airflowamount QT and the signal concerning the airflow amount Q from the flowsensor 7D, outputs the PIM actuation signal for converging the deviationbetween the target airflow amount Qt and the detected airflow amount Qupon zero to the unit 11D for outputting a control signal forcontrolling the power driving unit. The unit 11D for outputting acontrol signal for controlling the power driving unit, which has inputthe PID actuation signal QC, produces a PWM control signal QP based uponthe PID actuation signal QC, which is output to the power driving unit(PDU) 5. Subsequently, based upon the PWM control signal QP, the powerdriving unit (PDU) 5 drives the driving motor 7F in a rotatable mannerat driving current with a prescribed polarity and a prescribed dutyratio whereby the revolution number of the supercharger (S/C) isincreased one after another. This increases the airflow amount Q flowinginto the cathode inlet side of the fuel cell (FC) 3 toward the targetairflow amount QT one after another as shown in FIG. 5.

On the other hand, the unit 11E for setting a target air pressure, whichhas input the target airflow amount QT from the unit 11B for setting atarget airflow amount, outputs a signal concerning the target airpressure PT required for attaining the target airflow amount QT withinthe CV value of the backpressure control valve 8A to the unit 11F forfeedback-controlling the air pressure, the unit 11H for setting theopening the backpressure control valve, and the unit 11L for setting theopening of the pressure control valve, respectively.

The unit 11L for setting the opening of the pressure control valve,which has input the target air pressure PT, sets a target hydrogen gaspressure which is an appropriate pressure slightly greater than thetarget air pressure PT, and outputs a signal 6 for setting the openingof the valve required for attaining the target hydrogen gas pressure tothe unit 11M for outputting a control signal for controlling the openingof the pressure control valve. Then, the unit 11M for outputting acontrol signal for controlling the opening of the pressure control valveoutputs a driving signal D2 having a given polarity and a given dutyratio in order to PWM control the opening of the pressure control valve9B produced depending upon the signal δ for setting the opening of thevalve to the pressure control valve 9B. As described above, the pressureof the hydrogen to be supplied into the anode inlet of the fuel cell(FC) 3 is adjusted to be an appropriate pressure slightly greater thanthe target air pressure PT.

Here, in the flowchart of FIG. 4, based upon the change Δα in theaccelerator opening α with the time elapse or based upon the change ΔQTin the target airflow amount QT with the time elapse, the target powergeneration amount IT is judged whether it is changed or not, i.e., thecontrol unit 11 judges whether or not the fuel cell (FC) 3 is in thetransition period (5S). If the target power generation amount IT isjudged to be changed in Step S5, i.e., in the transition period, theairflow amount Q is subsequently judged whether or not it is convergedto the target flow amount QT (S6).

In the case, the result of the judgment in Step S6 is “NO”, which isassumed to be during the transition period that the airflow amount Qflowing toward the cathode inlet side of the fuel cell (FC) 3 isgradually changed toward the target airflow amount QT, the unit 11H forsetting the opening the backpressure control valve, the unit 11J forswitching the input, and the unit 11K for outputting a control signalfor controlling the opening controls the air pressure P at the cathodeinlet side to be a target air pressure PT one after anothercorresponding to the airflow amount Q, which is gradually changed.Specifically, the unit 11H for setting the opening the backpressurecontrol valve, which has input the signal of the target air pressure PTand the signal of the airflow amount Q from the flow sensor 7D, makes amap search for seeking the opening of the backpressure control valve 8Arequired for attaining the target air pressure PT one after anothercorresponding to the airflow amount Q, which is gradually increasedtoward the target airflow amount QT (S7), the signal y for setting theopening of the valve is output to the unit 11J for switching the input,while altering the signal γ for setting the opening of the valve at 10ms cycle. Subsequently, the unit 11J for switching the input outputs thesignal γ for setting the opening of the valve from the unit 11H forsetting the opening the backpressure control valve to the unit 11K foroutputting a control signal for controlling the opening, and the unit11K for outputting a control signal for controlling the opening outputsa driving signal D1 having a given polarity and a given duty ratio inorder to PWM control the opening of the backpressure control valve 8Acorresponding to the signal γ for setting the opening of the valve tothe backpressure control valve 8A, whereby the opening of thebackpressure control valve 8A is controlled to be γ (S8). In this case,the value of the signal γ for setting the opening of the valve is set tobe such characteristics that it is once decreased at the initial stageof starting the increase in the airflow amount Q as shown in FIG. 5,and, thereafter, it is increased according to the increasing of theairflow amount Q. For this reason, the air pressure P at the cathodeinlet side of the fuel cell (FC) 3 is increased one after anotherwithout once decreasing the pressure as in the case of the prior artshown as the broken line.

On the other hand, in the flowchart of FIG. 4, in the case where thejudgment in Step S5 is “No” or where the judgment in Step 6 is “Yes”,and the real airflow amount Q detected by the flow sensor 7D has reachedthe target airflow amount QT, the unit 11F for feedback-controlling theair pressure, the unit 11G for indicating the opening of thebackpressure control valve, the unit 11J for switching the input, andthe unit 11K for outputting a control signal for controlling the openingexecute feedback control of the opening of the backpressure controlvalve 8A so that the real air pressure P detected from the pressuresensor 7E is converged to the target air pressure QT. Specifically, theunit 11F for feedback-controlling the air pressure outputs to a unit 11Gfor indicating the opening of the backpressure control valve a PIDactuation signal PC for rapidly converging the deviation between thedetected air pressure P and the target pressure PT upon zero. Then,based upon the PID actuation signal PC, the unit 11G for indicating theopening of the backpressure control valve, which has input the PIDactuation signal PC, makes a map research for seeking an opening of thebackpressure control valve 8A required for obtaining the target airpressure PT within the CV value of the backpressure control valve 8A(S9), and the resulting signal β for indicating the opening is output tothe unit 11J for switching the input. Subsequently, the unit 11J forswitching input outputs the signal γ for setting the opening of thevalve from the unit 11H for setting the opening the backpressure controlvalve 8A to the unit 11K for outputting a control signal for controllingthe opening, and the unit 11K for outputting a control signal forcontrolling the opening outputs a driving signal D1 having a givenpolarity and a given duty ratio in order to PWM control the opening ofthe backpressure control valve 8A produced based upon the signal γ forsetting the opening of the valve, whereby the opening of the opening ofthe backpressure control valve 8A is controlled to be β (S10).

Specifically, according to the fuel cell system and the process forcontrolling the fuel cell of the present invention, during thetransition period that the airflow amount Q at the cathode inlet side ofthe fuel cell (FC) 3 is gradually changed toward the target airflowamount QT, corresponding to the change in the flow amount Q of the airflowing towards the cathode inlet side, which is detected one afteranother, the air pressure control means controls the air pressure P atthe cathode inlet side to be the target air pressure PT one afteranother. Consequently, the increasing of the pressure difference betweenthe poles within the fuel cell (FC) 3 can be prevented, ensuring theprevention of the damage of PEM making up MEA of the fuel cell (FC) 3.What is more, the time delay by which the air pressure P at the cathodeinlet side reached the target air pressure PT can be solved, improvingthe response to the increasing or decreasing of the power generationamount of the fuel cell (FC) 3.

While the embodiments of the present invention has been described, itshould be noted that the present invention is not restricted thereto andvarious modification can be made without departing from the scope andthe spirits of the present invention.

1. A process for controlling a fuel cell system comprising a fuel cell, which generates power by reacting anode gas and cathode gas supplied to the fuel cell, a compressor which varies a rotation number thereof to thereby control an amount of the cathode gas to be supplied to the fuel cell, and a pressure control valve which varies an opening thereof to thereby control a pressure of the cathode gas, wherein the pressure control valve is provided downstream of a cathode of the fuel cell, said process comprising: first controlling said compressor to change an amount of the cathode gas supplied to the fuel cell at a start of a transition period of said fuel cell, and thereafter changing an opening of said pressure control valve, depending on the changed amount of the cathode gas, to thereby regulate the pressure of the cathode gas, wherein an amount of power generated from the fuel cell is changed during the transition period, wherein the opening of the pressure control valve for controlling the pressure of the cathode gas is decreased during a first period of the transition period, and thereafter the opening of the pressure control valve is increased following an increase of the cathode gas flow amount.
 2. A process for controlling a fuel cell system comprising a fuel cell, which generates power by reacting anode gas and cathode gas supplied to the fuel cell, a compressor which varies a rotation number thereof to thereby control the amount of the cathode gas to be supplied to the fuel cell, and a pressure control valve which varies an opening thereof to thereby control a pressure of the cathode gas, wherein the pressure control valve is provided downstream of a cathode of the fuel cell, said process comprising: controlling a power generation amount of the fuel cell by first controlling the compressor to change the flow amount of the cathode gas at a start of a transition period of the fuel cell and thereafter controlling an opening of the pressure control valve to change the pressure of the cathode gas compressively transferred into a cathode inlet side of the fuel cell depending on the changed amount of the cathode gas, and controlling said pressure of the cathode gas to be a target gas flow amount corresponding to the detected gas flow amount, which is gradually changed, during the transition period of said fuel cell, wherein an amount of power generated from the fuel cell is changed during the transition period, wherein the opening of the pressure control valve for controlling the pressure of the cathode gas is decreased during a first period of the transition period, and thereafter the opening of the pressure control valve is increased following an increase of the cathode gas flow amount.
 3. The process as claimed in claim 1, wherein a pressure feedback control operation for controlling the pressure of the cathode gas to be a prescribed value is avoided in the transition period of the fuel cell.
 4. The process as claimed in claim 1, wherein the amount of the cathode gas is increased in the transition period.
 5. The process as claimed in claim 4, wherein the pressure of the cathode gas is increased during the first period of the transition period.
 6. The process as claimed in claim 5, wherein the pressure of the cathode gas is increased during a second period of the transition period following the first period of the transition period.
 7. The process as claimed in claim 6, wherein increasing the opening of the pressure control valve prevents an excessive increase in the pressure of the cathode gas during the second period of the transition period.
 8. The process as claimed in claim 1, wherein the amount of the cathode gas is increased to a target amount of the cathode gas at an end of the transition period.
 9. The process as claimed in claim 8, wherein if the amount of the cathode gas is increased to the target amount of the cathode gas, the pressure control valve is controlled based on a feedback pressure of the cathode gas so that the pressure of the cathode gas reaches a target pressure of the cathode gas.
 10. The process as claimed in claim 1, wherein a response of the compressor is slower than a response of the pressure control valve. 